The present invention relates to a television signal coder for encoding a television signal making use of correlations to compress the amount of information to be transmitted.
An example of the prior art systems for the transmission of a television signal by reduced amount of information based on the frame-to-frame correlarion is shown in an article by J. C. Candy et al entitled "Transmitting Television as Clusters of Frame-to-Frame Differences" published in the THE BELL SYSTEM TECHNICAL JOURNAL, July-August, 1971, pp 1889-1917. In this transmission system, the frame-to-frame difference, i.e., the difference in signal level between a picture element of one frame and that of the immediately preceding frame is derived and transmitted only when it is significant compared with a predetermined threshold value. Because of the high frame-to-frame correlation inherent to a television picture signal, this system makes it possible to reduce the amount of information to be transmitted significantly. However, when the signal is picked up from a fast changing subject, the frame-to-frame correlation is lowered, resulting in the increase of the amount of information to be transmitted. This may be overcome by quantizing the frame-to-frame differences and making the quantizing levels variable depending on the rate of the change in the image to be picked up.
However, the broadening of the quantization width results in a degradation of the quality of the reproduced picture with a granular noise appearing over the entire picture. This problem is avoided only by increasing the number of quantizing levels.